The present invention relates to an arrangement for determining the level in a container comprising a capacitive probe which is disposed insulated in the container and the capacitance of which depends on the level, a measuring transducer which is disposed at the location of the container and produces a measured value signal dependent on the probe capacitance and an evaluation means which is disposed remote from the container and which is connected to the tranducer by a two-wire line via which on the one hand the DC energy necessary for operating the transducer is transmitted from the evaluation means to the transducer and on the other hand the measured value signal furnished by the transducer is transmitted to the evaluation means.
Arrangements of this type have the advantage that between the measuring transducer and the evaluation means only a simple two-wire line is present which serves both for the current supply of the transducer and for the transmission of the measured value signals. The signal quantity designating the level is the frequency of the oscillations produced by the oscillator; measured value signals with this frequency or preferably with a lower frequency derived therefrom by frequency division are superimposed on the supply direct current on the two-wire line. The evaluation means determines the frequency of the superimposed measured value signals and obtains therefrom information on the level. It is frequently only necessary to determine the falling below or exceeding of a predetermined maximum or minimum level; such arrangements are then referred to as limit switches.
It is known from DE-OS No. 2,701,184 for the purpose of superimposing measured value signals to connect a resistance branch controlled in time with the measured value signals to the two-wire line in shunt with the measuring transducer. The measured value signals are then additional current pulses which are superimposed on the supply direct current carried by the two-wire line. When applying this feature to an arrangement of the type set forth at the beginning the recurrence frequency of the superimposed current pulses represents the level in the container.
It is also possible to convert the frequency of the oscillator to a direct current dependent thereon and to superimpose this direct current as measured value signal on the supply direct current along the two-wire line. The evaluation means is then constructed so that it responds to the changes in the direct current.
However, it can occur in the known arrangements of this type that interferences at the probe or in the transducer are not recognised or even interpreted as incorrect level. In certain uses a complete absence of the measured value signals, due for example to failure of the transducer by a probe short-circuit or by probe breakage, may even lead to an erroneous interpretation. For with dissipative filling materials it may happen that the vibrations of the oscillator stop due to the extreme damping when the probe is completely covered by the material. From the point of view of the level measurement this phenomenon is not disadvantageous because the absence of the oscillations can be interpreted as "container full" information. However, the problem remains that it is not possible to recognise in the evaluation means whether the absence of the measured value signals is a normal operating condition or the consequence of serious trouble.
The objective of the present invention is to provide an arrangement of the type described at the beginning in which errors and trouble occurring are reliably recognised in the evaluation means and distinguished from the normal operating conditions.
According to the invention this is achieved by a switching means disposed in the transducer for switching from the capacitive probe to a test capacitance which is different to any probe capacitance occurring in normal operation and by a control means for periodic actuation of the switching means.
In the arrangement according to the invention in each actuating period of the switching means two different types of measured value signals are transmitted, that is in one partial period (with the probe connected) measured value signals expressing the level information and in the other partial period (with the probe disconnected) measured value signals which correspond to the test capacitance and indicate the correct operation of the transducer. In this manner errors and trouble can be recognised in the evaluation means.
Since the probe capacitance is disconnected completely during the test partial period, the test signal is constant and in particular independent of the level in the container. Furthermore, it differs from any level measuring signal occurring in operation and consequently there is no danger of confusion.
The arrangement according to the invention is particularly suitable for measuring transducers which contain an oscillator whose frequency depends on the probe capacitance. In this case the oscillator generates for each periodic actuation of the switching means a test frequency which is different from the measuring frequency and depends on the test capacitance. This makes it possible in the evaluation means to recognise errors and trouble which change the oscillating frequency of the oscillator and thus indicate an incorrect level. Furthermore, even with dissipative filling material it can clearly be seen whether the absence of the oscillations is due to a failure of the apparatus or corresponds to the normal operating condition in which the probe is completely covered. For in the latter case the oscillator must start oscillating again in the partial periods in which the probe is disconnected so that in these partial periods a measured value signal with the corresponding recurrence frequency or the corresponding direct current value is transmitted via the two-wire line to the evaluation means. In contrast, in the former case the measured value signal is absent completely and with the arrangement of the invention this always indicates a fault.
An advantageous further development of the invention resides in that the capacitive probe comprises a probe electrode which is insulated from the container and via which a galvanic circuit controlled by the switching means passes, and that a detector circuit is provided which responds to the periodic current pulses produced by the periodic actuation of the switching means in the galvanic circuit and on absence of the periodic current pulses effects the transfer of information designating said absence to the evaluation means.
With this further development it is in particular possible to determine whether a short-circuit or an interruption is present in the probe connection. For in both cases the detector circuit receives no current pulses with the recurrence frequency of the actuation of the switching means but a constant potential.